How to reflect\catch light but be invisible on render?
I couldn't manage to make this "light catcher" using nodes but as a workaround or just alternate method you can use Baking. For this answer I'm using Blender 2.80 Beta and Cycles but the workflow is the same as in 2.79b.

Scene setup for baking




Background needs to be black.
Light should be fully white - you can change color of it after bake.
Object may be simple Diffuse - needs to be UV Unwrapped.


Baking



Bake Type - Diffuse with Influence Direct.

You should get a map like this:



Using it



Now you can use baked map as a mask to combine Transparent shader with another of your likeness. Color Ramp is for fine tuning mask strength.